The present invention relates to a lighting control circuit for head lamps in automobiles etc.
Usually, a head lamp circuit for an automobile is connected from a battery to head lamps, each having a high-beam filament and a low-beam filament, via a lighting switch as well as a dimmer switch, and it energizes either the high-beam filament or the low-beam filament in such a way that the dimmer switch is changed-over depending upon ambient brightness, the presence of a car running in the opposite direction, etc. (refer to, for example, the official gazette of Japanese Utility Model Registration Application Publication No. 41742/1977).
The Automotive Safety Standards of Canada contain the provision of Daytime Running Lamp (hereinbelow, abbreviated to "DRL") that, while an automobile is running, head lamps each having an illuminating area of at least 40 cm.sup.2 and a certain prescribed luminosity must be lit up even in the daytime.
In case of using the high beam of the head lamp, the certain prescribed luminosity signifies that luminosity of the central point (H-V point) of the optic axis of the high beam which ranges from 2,000 to 7,000 Cd inclusive. On the other hand, in case of using the low beam of the head lamp, the certain luminosity is stipulated in terms of a voltage value for driving the lamp, and the lamp must be lit up by the rated voltage thereof or by a voltage lowered to 75 thru 92% of the low-beam terminal voltage.
As the DRL stated above, it is considered to employ projector type auxiliary head lamps each of which is disposed separately from the conventional head lamp and is lit up simultaneously with the projection of a sub beam in order to enhance the light distribution performance of the low beam of the conventional head lamp. In many cases, however, the auxiliary head lamp does not satisfy the aforementioned condition of the illuminating area and cannot be used. Moreover, in the case of projecting the low beam of the head lamp with the rated voltage, the lifetime of the low-beam filament of the lamp shortens because this lamp is lit up even in the daytime. Therefore, it is common to employ a method in which a resistor is inserted in a lighting circuit for energizing the low-beam filament, whereby a voltage to be applied across the low-beam filament is lowered to 75 thru 92% of the terminal voltage of the low-beam filament.
The prior-art method stated above has the drawbacks that the loss of electric power which is consumed in the resistor is heavy and that the resistor is considerably expensive.